Bouncing Mail doesn't always do the trick. I've had some success with just killing the process, but YMMV.
My hypothesis is that it's HTML Mail rendering that's got it's knickers in a twist. When the process had been quit, the mesage I was reading was blank, but selecting another one and then coming back to the blank one ... it was rendered, and it was and HTML page.
Dunno if that helps anyone. Maybe it's enough to know you're not alone ?